and why people using tomcat is of course it is free as major factor, some other reasons like stability , easy to use ..it did a very good job considering if you only want to use tomcat for jsp and servlet processing, it running is light weight as compare with j2EE application server which consist other components that you may not need..

further points like supported and intergrated with major IDE , finally, it is extensible if you want to plugin some nice framework like struts